Hi, Well it's going to be a bit difficult to answer this question without more details. First, how old is this Pin Oak. It it's really large and produces a lot of shade underneath it, it will be very difficult to grow anything under its branches. I generally do not recommend trying to grow anything under Pin Oaks because they have such low sweeping branches and the soil underneath them can be very acidic. Now, as far as the insects are concerned, keep in mind that most insect pests are plant specific. I don't know of many insect pests that would dine on all three of the plant species you mention here. Plus, for control, you really need to know what kind of pest you are dealing with. My guess is that the problem really lies with the site. If your plants are weakened from the poor soil and light conditions under that oak, they will not thrive and might die. Never, ever spray your garden in a general manner because you will eliminate good insects as well as bad ones and the spray you use might not even be effective for the insects you have. Plus, if you spray the wrong product or in the wrong amounts, you can easily hurt the plants. So, start by analyzing the site and maybe you'll have much better luck growing plants away from the base of that tree.